MgSiO₃ perovskite is a principal mineral in the upper part of the lower mantle, but its stability and possible phase transition at greater depths have long been uncertain Recently, a new high-pressure MgSiO₃ polymorph called “post-perovskite” was discovered above 125 GPa and 2500 K on the basis of X-ray diffraction measurements in a laser-heated diamond anvil cell (LH-DAC). Crystal structure of post-perovskite was first determined to be orthorhombic (space group: Cmcm) by molecular dynamics (MD) calculations. The first-principles theoretical calculations also confirmed the stability of this new phase. These results suggest that MgSiO₃-rich post-perovskite is a predominant mineral below 2500–2700 km depth near the base of the mantle. The D″ layer has long been the most enigmatic region in Earth's interior. The post-perovskite phase can account for the large seismic anomalies observed in the D″ region, such as D″ discontinuity, polarization anisotropy, and anticorrelation between S-wave and bulk sound velocities. The long-term enigma may be explained with this newly discovered crystal.